In general, an image sensor is a semiconductor device that converts an optical image to an electric signal. Image sensors are generally classified as a charge coupled device (CCD) image sensor or a complementary metal oxide silicon (CMOS) image sensor (CIS).
In a related art image sensor, a photodiode is formed in a substrate with transistor circuitry using ion implantation. As the size of a photodiode reduces more and more for the purpose of increasing the number of pixels without an increase in chip size, the area of a light receiving portion reduces, so that an image quality reduces.
Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion also reduces due to diffraction of light, called airy disk.
In a horizontal type CMOS image sensor according to the related art, a photodiode and a transistor are horizontally formed adjacent to each other on a substrate. Therefore, an additional region for the photodiode is required, which may decrease a fill factor region and limit the possibility of resolution.
Also, the horizontal type CMOS image sensor according to the related art may cause a crosstalk problem between pixels.
Moreover, in the horizontal type CMOS image sensor according to the related art, it is very difficult to achieve the optimized process of concurrently forming the photodiode and the transistor.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a readout circuitry on a Si substrate and forming a photodiode on the readout circuitry using a method such as wafer-to-wafer bonding has been made (referred to as a “three-dimensional (3D) image sensor”). The photodiode is connected with the readout circuitry through a metal line.